Geographically, there are relatively few locations where a steady high wind prevails. Instead,.there are many more locations that enjoy a dependable low-speed wind. The prior art reflects numerous attempts by inventors to harness this bountiful energy resource.
In general, it is true that the design of a wind motor is based on and reflects the range of wind speeds at which it is intended to operate. Wind motors designed for use at high wind speeds typically employ propellers, and either do not work at all or are grossly inefficient at low wind speeds. On the other hand, machines designed for use at low wind speeds typically employ large sheetlike airfoils or sails, which are susceptible to damage at high wind speeds, and therefore have to be shut down when high winds are blowing.
A wind motor designed for use at low wind speeds must have a long maintenance-free life. Because the output power is relatively small, a long amortization time is implied. Accordingly, the wind motor must be designed to withstand exposure to the elements, and should be constructed of readily available parts. Also, a low-speed wind motor should be capable of unattended operation.
The prior art relating to low-speed wind motors appears to fall into several well-defined categories.
One category includes wind motors in which a number of sails are mounted for rotation on arms which rotate about an axis. U.S. Pat. No. 2,603,300 to King exemplifies this category, which includes in addition: U.S. Pat. No. 2,406,268 of Terhune; U.S. Pat. No. 1,964,347 of Ford; U.S. Pat. No. 1,577,096 of Aust, U.S. Pat. No. 1,321,415 of Brown; and U.S. Pat. No. 1,154,930 of Might.
Closely related to the first category of patents are a group of patents relating to aircraft propellers, exemplified by U.S. Pat. No. 2,045,233 of Kirsten, et al. In these patents, a number of airfoil blades are rotatably mounted to a wheel, with the individual axes of rotation parallel to the axis of rotation of the wheel. Other patents showing such propellers are: U.S. Pat. No. 1,636,423 of Langenbauch; U.S. Pat. No. 1,591,977 of Horne; and, U.S. Pat. No. 1,432,700 of Kirsten.
The most pertinent prior art is believed to be found in a category in which one or more sails are mounted on an oscillating arm. These patents will now be discussed at somewhat greater length.
In U.S. Pat. No. 4,595,336, Grose shows a wind motor in which an elongated boom is centrally pivoted for rocking movement in a vertical plane. Wings are mounted to the upwind and downwind ends of the elongated boom, and each wing includes a control tab or panel extending along its downstream edge. The control panels are movable with respect to the wings and influence the angle of attack of the wings. The positions of the control panels are altered by the snap action of a toggle as the rocking boom approaches the limit of its oscillating motion, thereby causing the arm to pivot in the opposite sense. There is no provision for altering the angle of the control panels in relation to the angle of the boom. Instead, the control panel is in either its maximum up position or in its maximum down position as determined by the toggle.
In U.S. Pat. No. 237,851, Foskett shows an elongated arm centrally pivoted for rocking movement in a vertical plane. At one end of the arm is a rudder which causes the arm to point into the wind. On the opposite end of the arm is mounted a frame in which a number of slats or vanes are pivotally mounted about parallel spaced horizontal axes. The angle of these vanes is abruptly altered at the end of each stroke by mechanical stops, in a first embodiment. In another embodiment, the angle is altered by an overcenter weight.
In U.S. Pat. No. 827,889, Smith shows a current motor having an arm pivoted about a vertical axis and a blade mounted to the arm for pivotal motion about a different vertical axis. The center of pressure of the blade must be upstream of the axis about which the blade pivots, so that the pressure of the flowing liquid will hold the blade in one or the other of its two extreme positions. The extreme positions are determined by a chain or cable whose effective length varies throughout the stroke in a determinable manner. There is no provision for rendering the device current-seeking.
In U.S. Pat. No. 317,756, Flint shows an elongated arm mounted for rotation in a horizontal plane, and having two vanes mounted in a plane and pivotally attached to the downstream end of the arm for pivotal motion about a vertical axis. Although the vanes are described as "hanging free and independent," the arm is said to oscillate in the horizontal plane. In operation, the vanes are described as altering between two extreme positions.
Thus, although a number of approaches have been tried to extract energy from low-speed winds, it appears that the inventions which employ oscillating arms also employ abrupt alteration of the angle of the sails at the end of each stroke. This abrupt change necessarily puts stress on certain elements of the device, and ultimately shortens its life.
As will be seen below, the apparatus of the present invention delivers its power in a smooth and controlled manner, thereby prolonging the life of the device.